Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
Two common types of flash memory array architectures are the “NAND” and “NOR” architectures. These architectures are named for the resemblance that the basic memory cell configuration of each architecture has to a basic NAND or NOR gate circuits, respectively.
FIG. 1 illustrates a simplified diagram of a typical prior art NAND flash memory array. The memory array of FIG. 1, for purposes of clarity, does not show all of the elements typically required in a memory array. For example, only two bit lines are shown (BL1 and BL2) when the number of bit lines required actually depends upon the memory density. The bit lines are subsequently referred to as (BL1-BLN).
The array is comprised of an array of floating gate cells 101 arranged in series strings 104, 105. Each of the floating gate cells 101 are coupled drain to source in each series chain 104, 105. A word line (WL0-WL31) that spans across multiple series strings 104, 105 is coupled to the control gates of every floating gate cell in a row in order to control their operation. The bit lines (BL1-BLN) are eventually coupled to sense amplifiers (not shown) that detect the state of each cell.
In operation, the word lines (WL0-WL31) select the individual floating gate memory cells in the series chain 104, 105 to be written to or read from and operate the remaining floating gate memory cells in each series string 104, 105 in a pass through mode. Each series string 104, 105 of floating gate memory cells is coupled to a source line 106 by a source select gate 116, 117 and to an individual bit line (BL1-BLN) by a drain select gate 112, 113. The source select gates 116, 117 are controlled by a source select gate control line SG(S) 118 coupled to their control gates. The drain select gates 112, 113 are controlled by a drain select gate control line SG(D) 114.
A selected word line 100 for the flash memory cells 130-131 being programmed is typically biased by programming pulses that start at a voltage of around 16V and may incrementally increase to more than 20V. The unselected word lines for the remaining cells are typically biased at Vpass. This is typically in an approximate range of 9-10V. The bit lines of the cells to be programmed are typically biased at 0V while the inhibited bit lines are typically biased at VCC.
As NAND flash memory is scaled, parasitic capacitance coupling between adjacent memory cell floating gates becomes a problem. Because of the capacitive coupling, the cells that are adjacent to a cell storing a charge are prone to having their threshold voltages (Vt) raised. If the adjacent cells have their threshold voltages raised too high, an unprogrammed cell might appear as being programmed. Due to floating gate-to-floating gate interference, a cell Vt of a source side cell is higher than Vt during a verify operation. For example, Vt may be 0.8V at verify but after the entire page of cells has been programmed, Vt may be 1.2V. These effects generate a wider Vt distribution when the distribution is needed to be tighter.
Additional NAND memory array problems are series string resistance issues and source resistance issues. String resistance increases from a programmed cell in the same string. This reduces the cell current and increases Vt for unprogrammed cells.
Similarly, increased source resistance creates a higher voltage drop per cell at the source wiring. Cell current is reduced and Vt is increased for unprogrammed cells.
These problems are illustrated in FIG. 2 that shows a memory programming method with the resulting floating gate-to-floating gate interference and the string resistance. This plot shows the Vt shifts for each word line (i.e., WL0-WL31) and a reason for a particular shift. For example, page 3 shows that the Vt shift was caused by floating gate-to-floating gate interference along the bit line as well as the string resistance. The dotted line 201 shows the original Vt and the solid line 202 shows the worst case shifted Vt after all pages have been programmed.
The above-described problems for single level cell (SLC) NAND arrays are even worse in a multiple level cell (MLC) array. MLC memory stores multiple bits on each cell by using different threshold levels for each state that is stored. The difference between adjacent threshold voltage distributions is typically very small as compared to an SLC memory device. Therefore, the effects of the floating gate-to-floating gate coupling in an MLC device are greatly increased.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a way to minimize the effects of coupling, string resistance, and source resistance in a memory device.